RHS Head Swap - LQ=HP

A Bumpstick Swap and a Set of RHS’ New Heads Make a Truck Motor Really Haul.

It didn’t take long for gearheads to figure out that GM’s Gen III truck engines are a great bang-for-your-buck buy for transplantation into a classic muscle car. There are differences, of course, but these powerplants are essentially iron Corvette motors, waiting for a few well-chosen upgrades to turn them into slick street machine motivators. That’s pretty much what we did here—found a truck motor that’s destined to reside in the engine bay of a ’66 Chevelle, and outfitted it with a new COMP Cams camshaft and a set of RHS’ new Pro Action cylinder heads with Clean Cast Technology, which is designed to create “as cast” ports that flow like those in ported heads. The results were impressive—and impressively inexpensive, to the tune of a measly $1,470 clams for a fully assembled pair of lungs. Yes, you read that correctly. Now read on.

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The foundation for this project was a 6.0L LQ9 powerplant. This variation of the Gen III family was found in ’02-06 Cadillac Escalades, ’03-07 Chevrolet Silverado SS models, ’04-05 Silverado and GMC Sierra “Vortec HO Editions”, and ’06-07 Silverado and Sierra Vortec Max optioned vehicles. The LQ9 achieves its factory-listed 345 hp and 380 lb-ft of torque by incorporating flat-top pistons to create a 10:1 compression ratio, compared to the 9.4:1-compression LQ4’s 300-325 hp and 360-370 lb-ft of torque. Most importantly, these things can be had cheap. We found one on eBay, complete with harnesses, computers, and a warranty for $2,500. Locally, we’ve found them for half that. You can start a muscle car project with a complete, running engine for as little as $1,200, which is hard to beat.

Our first modification was to scrap the factory intake manifold and fuel-injection system for a simpler and proven carbureted setup, featuring an Edelbrock LS1 Performer RPM intake manifold, MSD 6LS timing module, and a Holley Ultra Street Avenger 770-cfm vacuum secondary carburetor. This is a quick install that we’ve covered before; it works and it’s where we started our day’s testing at Westech Performance. The initial results were what you’d expect from a factory truck motor: lots of torque (400-plus lb-ft all the way to 5,400 rpm), not much in the horsepower department (416 peak ponies at 5,700 rpm, where the stocker is done revving). It’s made to haul loads of whatever at uninspired speeds, not for street/strip warrior duty.

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The second modification is certainly the most important: a cam swap, which gave this engine a total personality transplant. “The Gen X engines seem to respond to cam changes so favorably that even the smallest cam can make a big difference,” Westech’s Steve Brulé says. “The heads are so efficient; they’re just waiting for a cam change.” Our testing certainly proved that point, but we were still surprised when we found the stock cam specs on LS1Tech.com; can you say broomstick cam? A switch was in order, though restraint was needed. The LQ9 has flat-top pistons, so attention must be paid to lift and duration when choosing a cam so proper piston-to-valve clearance is maintained. That being said, the cam of choice turned out to be a COMP XR281HR, a piece from the company’s XFI RPM Hi-Lift line. Though COMP does make bigger cams that will fit this application, these ’sticks are still aggressive for an LS application. Which in this case is a good thing. The transformed truck motor recorded a sharp increase in horsepower, as you’d probably expect. It also makes its 495 hp up at 6,300 hp—right around the place a Corvette engine touches its best. Perhaps less expected, however, is the much fatter torque curve, which peaks at a lower rpm (460 lb-ft at 5,100 rpm), and spreads its grunt over a much broader range. It’s a testament to how critical cam choice is, and also to how good the factory heads are. We could have quit here and had a very powerful and cool street/strip powerplant. On the other hand, there was no way we were leaving any ponies on the table.

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RHS Head Swap - LQ=HP

In short, the LQ9 is a high-output version of the LQ4 engine found in a multitude of Chevy and GMC heavy-duty trucks. The 6.0L (370ci) LQ9 achieves its factory listed 345 hp and 380 lb-ft of torque by incorporating flat-top pistons to create a 10.0:1 compression ratio, compared to the 9.4:1-compression LQ4’s 300-325 hp and 360-370 lb-ft of torque. (Photo courtesy of Chevrolet.)

Although our donor truck engine was in fantastic shape, it’s still a used motor, so we drained the dead dinosaurs out of the sump and refilled it with a fresh helping of AMSOIL 10W30 synthetic and a new K&N filter prior to our dyno session.

With the engine mounted on the testbed, we could step back and take a look at our starting point. The factory truck intake was replaced with what is now a tried-and-true setup: Edelbrock’s Performer RPM LS1 intake manifold and MSD Timing Module, and a Holley carb. These mods alone on an otherwise stock truck mill showed significant power gains over the stock EFI setup.

We had to swap out the stock valvesprings to handle the massive increase in lift provided by our new cam. We went with the PN 26918-16 beehive valvesprings. The stock heads proved they are capable when paired with a healthy camshaft, though there was still a decent chunk to be gained with our new RHS heads.

As we mentioned, the LQ9 gets its compression boost over the more common LQ4 from a set of flat-top pistons. Given the lack of piston reliefs, and the fact that the slugs protrude 0.009-inch above the deck (thanks again, LS1Tech.com), a new cam must be chosen with care. Also, be sure to blow any water out of the block boltholes when swapping heads, or you’ll have problems with the new head bolts.

You usually can’t see the difference in camshaft specs by looking, but the disparity between the stock cam and our COMP replacement is evident to the eyeball. The stocker specs in at 0.497/0.467-inch lift (intake/exhaust) and 196/207 degrees duration at 0.050 with an LSA of 116 degrees. The XR281HR we chose has more than 0.074-inch extra lift (it’s a whopping 0.106 inch on the exhaust side), along with more than 30 degrees extra duration on both sides. It isn’t the biggest LS cam in COMP’s catalog, but it positively transformed our pedestrian truck engine.

These heads use RHS’ Clean Cast Technology, which uses permanent-mold tooling to closely control manufacturing tolerances. This control goes a long way toward providing the tight tolerances you’ll find in a CNC’d head, without the expense of a full port job. As is typical with aluminum heads, you can see the cleanup work done inside the port.

A comparison of the stock (left) and RHS intake ports shows the difference between 205 cc and 225 cc of intake runner area in the new heads. RHS’ Kevin Feeney says, the flow numbers are conservative, as the RHS engineers focused more on the quality of the airflow, rather than just quantity. If the air speed is hindered, low-end torque suffers. Based on our numbers, we’d have to say this isn’t a problem with these heads, as our creation has plenty of low-end pull along with gobs of top end power.

The combustion is also much different than the OE chamber. It’s got more of a kidney bean shape, as opposed to the heart-shaped stocker. It’s also designed to “improve airflow and flame propagation”, which leads to increased power and torque. Again, we’d have to say that our numbers bear out the claims. It also bumped the compression up a bit, from 10:1 to 11.41:1—we had to back our timing off by 1 degree, but still made great power on California’s tepid 91-octane gas.

And finally, we outfitted our engine with Edelbrock’s latest aluminum ignition coil covers. The factory coils bolt directly to the cover, eliminating the factory bracket. They look great, but this engine still looks better than it runs, which is how it should be. CHP